Why does 'append' mutates all elements in a list?

Question

I am building a list of lists. I am reading the elements from an input file. I am reading in each line in the file as a string-single-element to a sub-list in the list. First, I created the list of lists:

>>> b = [[]] * 5

However, when I tried to append an item, I got the following:

>>> b[1].append("abc")
>>> b
[ ['abc'], ['abc'], ['abc'], ['abc'], ['abc'])

Why does append change all the sub-lists? Is insert() better in this situation?

problem is [[]] * 5 - see what happens with this b = [[],[],[],[],[]] — user3125280, Jan 02 '14 at 01:59
see here http://stackoverflow.com/questions/6688223/python-list-multiplication-3-makes-3-lists-which-mirror-each-other-when — corn3lius, Jan 02 '14 at 02:00

Ankur Ankan · Accepted Answer · 2014-01-02T02:05:18.950

10

When you do [[]] * 5 it is a list of same objects (empty lists here) repeated 5 times.

You can check it by:

>>> b = [[]] *5

>>> b
[[], [], [], [], []]

>>> id(b[0])
140588316211896

>>> id(b[1])
140588316211896

What you would need to do is:

>>> b = [[] for i in range(5)]
>>> b[0].append('abc')
>>> b
[['abc'], [], [], [], []]

In case of [[] for i in range(5)], in each loop a new empty list object is created.

And insert would not be a good option here because insert is a O(n) operation whereas appending would be O(1).

edited Jan 02 '14 at 02:05

answered Jan 02 '14 at 02:00

Ankur Ankan

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Could you please explain why your solution works, why in your soultion, the lists are not the same object? – idish Jan 02 '14 at 02:06
@idish: Because in every loop a new empty list object is created. – Ankur Ankan Jan 02 '14 at 02:07
@ Ankur How do you know insert() is O(n) and append O(1)? – Concerned_Citizen Jan 02 '14 at 04:14
@Dombey: You can check the time complexity of various python methods here: https://wiki.python.org/moin/TimeComplexity – Ankur Ankan Jan 02 '14 at 07:02

score 1 · Answer 2 · answered Jan 02 '14 at 01:59

1

When you say

b = [[]] * 5

It creates 5 lists that all point to the same thing. If you print out the address of each element you'll find that they are the same.

So it wouldn't matter whether you used append or insert.

answered Jan 02 '14 at 01:59

MxLDevs

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No. Under the hood, the list is primarily composed of 5 references to the same unique list ``[ ]``, it means nothing to say a list points to a "thing" . A thing ? There's a word for that, it's "object". – eyquem Jan 02 '14 at 02:18
@eyquem Yes, it would not be accurate to say it is a list of lists, but I don't really see much of a difference between referring it as some "thing" or some "object". – MxLDevs Jan 02 '14 at 04:25

score 0 · Answer 3 · answered Jan 02 '14 at 02:01

0

This is a FAQ. All 5 top-level list elements are pointing at the same list.

Check this out:

#!/usr/local/cpython-3.3/bin/python

if True:
    b = [[] for dummy in range(5)]
else:
    b = [[]] * 5

b[1].append("abc")

print(b)

answered Jan 02 '14 at 02:01

dstromberg

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Why does 'append' mutates all elements in a list?

3 Answers3